As well paths in exploration and extraction activities in mining industries become increasingly longer and the network more complicated, new challenges are constantly being faced in the area of well drilling.
The cost of drilling a well is generally dependent on time—the longer it takes to drill the well the more expensive are the well establishment costs. It is therefore highly desirable to establish a well in the shortest time possible.
A major factor which contributes to the cost of a well relates to the well trajectory i.e. the path the bore will take between the surface and the reservoir. Whilst the length of the required path is set, optimizing the actual well trajectory to follow the desired path is of great importance.
Wells can often be thousands of metres long and require drilling through earth formations which can vary greatly in relation to their geological properties. Furthermore the forces that are placed on the drill bit, and those experienced by the bottom-hole assembly (BHA) and the whole drill string affect the evolution of the bore. These factors cause the actual trajectory of the well to deviate from the desired trajectory requiring the operators to constantly monitor the trajectory and often make corrections to the drilling direction. Such corrections can be made using a remotely controlled steerable system. However, these systems require further consideration by the drilling operators.
In worst case scenario, if it is not possible to correct the trajectory, the well must be plugged and restarted. Furthermore, an inaccurate trajectory can also result in loss of the drill string.
Corrections obviously affect the uniformity of the well, creating crookedness/dog legs, side tracks and overgauging of the bore diameter. This not only increases the time taken to drill the well but also creates problems when inserting and fixing casings within the well and alters the shape of the well wall. All these factors add to the expense of drilling the well, and may reduce the performance and life of the well.
As directional drilling has become more important over recent times various models and methods of gauging the actual trajectory of a drill bit have been developed. To date these methods assist in predicting and monitoring well trajectory but still require further corrective manipulation to provide worthwhile results.
Prediction of well trajectory, design of bottom-hole assembly (BHA), and control of rotary steerable systems must rely on a mathematical model to quantify the parameters which affects the borehole trajectory. The model accounts for:                the elastic response of the drill string,        contacts between the stabilizers (and possibly other parts of the BHA and the drill pipes) and the borehole wall, and        the interaction between the bit and the rock.        
As a result of the large number of parameters involved in the drilling process this model turns out to be non-classical, in particular in view of the bit-rock interaction. Whilst not all parameters have a significant effect on the trajectory it is so far not certain which parameters must be considered in order to accurately predict the actual well trajectory. Certainly the prior art models do provide useful theoretical predictions, however, trajectory inaccuracies still occur when drilling the well.
When drilling, a drill bit is forced to engage and cut into the rock by the weight acting on the bit, with the debris removed by the injection of high pressure drilling fluid through the bit. Drill bits come in a multitude of configurations to suit different conditions. However, information regarding the behavior of the bit is not readily available and therefore it is difficult to incorporate the drill bit behavior and its interaction with respect to the rock when planning and drilling the well.
A vast proportion of current methods and models consider the weight and forces experienced by the drill string without considering the interaction of the drill bit with the rock.
A large portion of the early work in relation to drilling wells and the effect various forces and assemblies have during that process were carried out in the 1950's by Lubinski, A. and Woods, H. B. The models suggested by these early innovators were quite simple. However, as the requirement for more complicated wells has increased these earlier works have been improved upon and further parameters are now taken into consideration. Once the importance of a previously unknown parameter (or a previously considered unimportant parameter) is identified it is very easy to realise the effect that parameter has on the well trajectory simply by not accounting for it in the modeling process.
A current model used in relation to predicting well trajectories is described in the publications “Prediction of Drilling Trajectory in Directional Wells Via a New Rock-Bit Interaction Model”, SPE Ann. Conf., Paper #16658, 1987 and in “General Formulation of Drill string Under Large Deformation and its Use in BHA Analysis”, SPE Annual Technical Conf. and Exh., October 1986, both by Hwa-Shan Ho. These publications go some way of taking into consideration the interaction between the bit and the rock, as well as side forces generated by the BHA.
This model was further described in U.S. Pat. No. 4,804,051, to Hwa-Shan Ho. This patent describes a model which accounts for a parameter(s) relating to the interaction of the bit and the rock, particularly taking into account/predicting the walk tendency of a given BHA. This model introduces the “anisotropy index,” which is the ratio between lateral drillability to axial drillability, and the walk angle—the angle between the lateral force and the lateral displacement.
Theoretical methods to compute the bit anisotropy index (renamed bit steerability) and the walk angle from simple bit geometrical parameters have been disclosed in two publications: S. Menand, H. Sellami, C. Simon, A. Besson and N. Da Silva, “How the bit profile and gages affect the well trajectory”, in Proceedings of IADC/SPE Drilling Conference held in Dallas, February 2002, IADC/SPE 74459; and S. Menand, H. Sellami and C. Simon, “Classification of PDC bits steerability according to their steerability”, in Proceedings of IADC/SPE Drilling Conference held in Amsterdam, February 2003. IADC/SPE 79795. In these publications, the curvature of the bore is considered to be inversely proportional to an arbitrary length, which in practice is chosen to be about 10 meters.
Finally, the above models from Ho and Menand et al. take into account the effect of rock anisotropy only through its effects on the side force on the bit.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.